As moons go, Europa is doing pretty well in the looks department. While other wrinkled and pockmarked planetary bodies look their age, Jupiter’s moon, despite being billions of years old, is one of the smoothest objects in our solar system.

Advertisement

However, this moon is far from flawless. Europa is suspected to have a perpetually dark, liquid water ocean enclosed beneath a thick shell of water ice – around 40 per cent of which is covered with long, dark scratches and scars.

The prospect of liquid water places Europa near the top of the list of places in our solar system that might host alien life. However, it is hard to know what’s actually going on in the sub-surface ocean. Does it teem with alien microbes – perhaps even bigger creatures – or is it a vast, inky, sterile wasteland?

The only window we have on the ocean is its icy surface, so scientists try to read its criss-cross scars for clues. But whether this so-called chaos terrain can tell us about what’s going on underneath such a thick layer of ice is hotly disputed.

Oceanic chaos

The criss-cross pattern is likely to be caused by warmer and thinner regions of ice breaking and refreezing. It is much more abundant around the moon’s equatorial regions than at its poles – but why should this be the case?

Unlike on Earth, the temperature difference between the equator and poles cannot be explained by the effects of the sun because its light is too faint and Europa’s surface too reflective.

Another theory involves Jupiter’s gravitational pull, which would produce tectonic forces and heat up Europa’s ocean – but models have shown that this would heat the poles more than the equator.

The latest study suggests that turbulence in Europa’s ocean sculpts the chaos terrain on the icy surface. It was previously assumed that an effect caused by the moon’s rotation – known as the Coriolis force – dominates the ocean’s flow, funnelling heat to high latitudes. The new model instead relies on ocean currents caused by convection of the moon’s internal heat.

Mix for life

The team found that this produces a chaos terrain very similar to the one seen on Europa. “The resulting flow is less organised, but more vigorous in the equatorial region,” says researcher Johannes Wicht of the Max Planck Institute for Solar System Research in Lindau, Germany. “This correlates nicely with the distribution of chaos terrain.”

The model suggests that the ocean is extremely turbulent, with three strong ocean jets. So despite the thickness of Europa’s icy shell, it seems that properties of its ocean are writ in the ice. “We may be able to understand Europa’s ocean just by looking at the surface,” says Wicht’s colleague Britney Schmidt of the Georgia Institute of Technology in Atlanta.

A turbulent ocean would be beneficial for any life there because it would help shift nutrients from the sea floor into the rest of the ocean, says Schmidt’s other colleague, Krista Soderlund of the University of Texas at Austin. Microbes can live in stagnant water, but knowing the ocean is turbulent makes life much more likely.

Studies of Europa’s chaos terrain may have relevance beyond Jupiter’s moon. “Icy subsurface oceans may be commonplace in the outer solar system,” says Leigh Fletcher of the University of Oxford, a member of the ESA Science Working Team for JUICE. “JUICE will also search for any active plumes and vents, just like on Saturn’s moon Enceladus, to offer a glimpse into this icy ocean. This will be a great test of this sort of model for the icy worlds of our solar system.”